Mary was saved from her own sins just as we are, through “grace by the merits of Christ our Lord and God, her Savior” as you phrased it so clearly. Without the substitution of Christ’s perfect sacrifice, even one sin falls short of the perfect standard and merits the just penalty of eternal separation from God. Jesus paid that penalty for Mary, too.
I couldn't have known this about this dear girl, God's lowly handmaid, apart from Scripture, that is, divine revelation: in this case, the highly revealing title by which God's angel addressed her.